1. Field of the Invention
The present invention relates to an integrated system for reading and writing information on magnetic recording media and more particularly to the implementation of read window margining and write precompensation in a magnetic disk drive controller system. Still, more particularly, the present invention is directed to an integrated system implemented in a final configuration computer system in which the dual functions of accelerated testing of the system to determine error rate and precompensation of write data can be performed with the same integrated hardware.
In recording, magnetic dipoles contained in the disk medium move past a recording head, which consists of an electromagnet with highly focused fringing field. The magnetic field due to the recording head current aligns the dipoles in one direction or another representing digital bits (logic 1's and 0's). Each bit occupies one bit cell. The magnetic field of the magnetic head extends somewhat over several bit locations. As shown FIG. 1A, the magnetization 10 of a track of the disk is shown as a function of position along the track, for a sharp rectangular pulse 12 of record head current. The resultant magnetization 14 due to adjacent record head current pulses 16 and 17 of opposite polarity is shown in FIG. 1B, from which it is seen that the overlap of the magnetization 18 and 19 due to the respective pulses 16 and 17 causes the adjacent magnetization peaks 20 and 22 of opposite polarity to be shifted from their respective center positions 24 and 26. This shifting due to interaction of neighboring bits is referred to as "peak shift". The amount of peak shift is greatest on the inner disk tracks, where bit spacing is smallest. To eliminate the effects of peak shift in subsequent data recovery, the write data is typically precompensated during the record process for peak shift, by judiciously advancing or delaying the write signal depending on the data pattern and the track radius. This process is referred to as "write precompensation".
A data signal is normally recorded on magnetic disks in encoded form consisting of data information as well as synchronous clock information of the rate at which the digital bits are written onto the disk. Ideally, the clock rate of data is a known fixed value. Due to various factors, however, such is not the case, and the clock rate of the data must be determined by looking at the data signal itself in order to accurately read the data from the disk. The encoded clock information is employed in the data recovery process to accurately determine the data rate when signals for the disk are being read.
During recovery of recorded data signal, the clock information is first recovered from the encoded data signal by a data separator. It is the transitions from logic 1 to 0 or vice versa at the boundary between bit cells, not the sense of the digital bits, that are essential to decoding. Referring to FIG. 2, typically, the signal recovered from a disk read is used to form narrow pulses 32, 34 wherein the rising edge 31 of each pulse corresponds to each transition, i.e. transition pulses which represent either data pulses 32 or clock pulses 34. A phase-locked loop driven by the transition pulses provides a recovered clock signal 28 which is equal to the clock rate of the data being read. The clock signal defines the bit cells 36. A read window signal 30 is generated based upon the recovered clock to distinguish the data pulses occurring near the center of bit cells from the clock pulses occurring near the edge of bit cells. In this fashion the data information can be separated from the clock information. Typically, the window signal is obtained by delaying the clock signal by h, a quarter of the clock period. This delay is often referred to as a "half window" delay. A commonly used data format is modified frequency modulation (MFM) format. The encoding and decoding sequence of this format is described in detail in copending U.S. patent application Ser. No. 803,664 filed on Dec. 2, 1985 and assigned to the same assignee as the present invention.
A data detector utilizes the read window signal to detect data pulses falling near the center of each bit cell. Under ideal conditions, the read window signal is in phase with the transition pulses generated from the recovered data signal from the disk such that each data pulse is located at the center of the window. However, when the data signal is exposed to interference in pulse timing, the transition pulses move away from the center of the read window due to a shift in phase, as illustrated in FIG. 3A. Pulse timing may fluctuate due to a number of factors, including magnetic surface flatness variations, variations in uniformity of the magnetic properties of the media, speed variations, wow and flutter, uncompensated second order peak shift effects, imperfect peak shift compensation, deep magnetization of the media, interference from magnetic patterns on adjacent tracks, incomplete erasure of previous recordings, magnetic noise, and electrical noise. If a transition pulse is shifted so much that it moves outside the edges 40 and 42 of the read window, as shown in FIG. 3B, that pulse will not be detected properly, thus giving rise to a read error. The read window thus defines the boundaries within which transition pulses corresponding to data can be properly detected even when there is drift in phase of the pulses.
Aside from removing the above-mentioned causes of transition timing problem, one method of preventing read errors is to increase the width of the read window within practical considerations. It is therefore desirable to evaluate the performance of a disk drive read function by estimating the error rate for a particular read window size. It is useful to find out the probable worst case of drift of transition pulse within the read window. Referring to FIG. 4, the time difference m between the boundaries 44 and 46 of the read window and the predicted worst case of drift of the transition pulse is the "read window margin". Read window margin is a valuable criterion in evaluating the performance of a digital magnetic recording system.
2. Description of the Prior Art
Typical disk record electronics providing for write precompensation function are shown in FIGS. 5A and 5B. The data pulses and clock pulses are encoded by an encoder 50 into a combined write signal. Referring to FIG. 5A, during write precompensation for peak shift, the phase of the write signal is shifted by a timing compensator 52 provided between the encoder 50 and a write head 54. A typical timing compensator is illustrated in FIG. 5B and includes three phase shifted signal lines 56, 58 and 60 representing early, normal and late write signals respectively from an array of delay-lines 62. A multiplexer 64 is employed to advance or delay the write data by selectively passing one of the phase shifted signals to the write head 54. Thus, the write signal is precompensated during the write process for peak shift by judiciously advancing and delaying the write signal as appropriate. On data recovery, the effect due to peak shift is thus reduced.
In the data recovery process, a common technique employed to study error rate is window margining analysis. Error rate is defined as the number of data pulses read before one detection error is encountered. For example, a 10.sup.-10 error rate means that on average one error will occur every 10.sup.10 pulses. If one measures timing errors with respect to different window sizes, the major component of the result typically corresponds to a guassian (or normal) distribution which may be represented by a probability density function 86 as shown in FIG. 6. For the purpose of margin analysis, it is more appropriate to present the probability density function on a logarithmic scale. Referring to FIG. 7, which shows a typical read detection error probability density function of a disk drive, the vertical axis represents error rate in logarithmic scale. In this particular example, the graph indicates that given a window size of 40 ns, a read error will occur in every 10.sup.10 transition pulses read on average. In other words, once in every 10.sup.10 transition pulses, a pulse will actually be read more than 20 ns away from the center of the window on one side. That is to say that a disk drive with this probability density function and a 10.sup.-10 error rate will be required to tolerate at least 20 ns of phase drift of the pulse from its nominal center position. If instead a window size of 60 ns is employed, there will be a read window margin of m=10 ns on each side of the window for a drive with a 10.sup.-10 error specification.
It is noted in FIG. 7 that beyond about 30 ns in window width (plus or minus 15 ns from the center of the window), there is expected a linear relation between window width and logarithmic error, as shown in region 88 of the graph. Thus, it is possible to deduce the error rate beyond window width of 30 ns from several measurements in the linear portion of the graph and then by extrapolation of the data. This is extremely useful because at low error rates, it is very time consuming to measure one occurrence of error in every 10.sup.10 pulses, for example, several times to obtain a meaningful average and to repeat the process for each increment of window size.
In practice, an external off-line test instrument dedicated to the purpose of window margin analysis is employed to analyze the read error performance of a disk drive system. Referring to FIG. 8, the test instrument is commonly connected to the disk drive system in a configuration such as to by-pass the on-board data detector 90. The test instrument includes a dedicated variable window decoder 92 which typically has a delay-line and a PLL system (not shown) incorporated in its circuit to selectively shift the phase of the window signal relative to the nominal position of the transition pulse signal. The transition pulses detected will therefore be closer to the edge of the window which was shifted toward the nominal position of the transition pulse signal. By shifting the window, the error rate is artificially increased since effectively the window width is decreased and the pulse is more likely to drift beyond the window edge. By shifting the window in increments and measuring an average read error rate each time, it is possible to generate a read error rate probability distribution plot similar to the one in FIG. 7, and extrapolate the plot to determine the system error rate in the absence of artificial window shifting, in much less time than would be required if no shift were employed.
In practice, disk drive manufacturers test the disk drives in relatively clean and electrically noise-free environments. The test equipment comprising a variable window decoder is complicated and expensive in order to achieve such test conditions. Thus, it is uneconomical for a manufacturer to tieup test equipment time for the testing of relatively inexpensive disk drives such as floppy disk drives commonly used in personal computers. It is clearly not feasible for end-users to maintain such expensive equipments. Another disadvantage of using an external test equipment is that the on-board normal data detector is not being used during the testing, which means that any imperfections in it are not being tested. Furthermore, the circuitry that is used in the test equipment may introduce its own imperfections. For example, it is possible that in the variable window decoder, its window sliding function interferes with the operation of its PLL. Moreover, the environment in which a disk drive is tested by the manufacturer is different from that in which the drive is actually put in service. The read window margin observed in a manufacturer's test bench can be substantially eroded by factors over which disk drive manufacturers have little control once a drive leaves a manufacturing plant. For example, in the final computer configuration, the presence of extraneous electrical noise from other hardware components such as disk controller, switching power supply, printer, monitor, keyboard and the like, coupled with a poor on-board decoder can reduce the read window margin compared to the value obtained in a drive acceptance test.